Among the dimensional inspections that are normally carried out on a camshaft for an internal combustion engine, in addition to the simple dimensional and geometric characteristics of the main journals and the cams, nowadays there are also those for checking other parameters of the cam profile as, for example, the slope and the slope variations of the profile. Inaccurate values of these parameters may, in fact, affect the running of the engine where the shaft is fitted, and cause, in particular, the improper opening and/or closing of the associated valves. In turn this may produce, for example, violent impacts between valves and valve seats, thereby causing noise, wear and damage to the valves and/or the detrimental-emission of combustion components and consequently the dispersion of pollutants into the atmosphere.
In order to be in a position to fully evaluate the above parameters, it is necessary to gain information pertaining to the profile trend of each cam.
A method and a device for measuring the profile of a cam are disclosed in British patent GB-A-1273479, wherein the radial dimensions of a cam placed in rotation are measured at specific angular positions and a suitable device controls the comparison made between measured values and corresponding desired values. The angular position of the cam with respect to a theoretical cam having the desired values is determined at first on the basis of a comparison, not described in detail, between a desired and a measured value.
In order to determine with greater accuracy the angular position of a cam there are also known devices as those described in British patent GB-A-1320929 and U.S. Pat. No. 3,731,386 wherein the angular position of the lobe of a cam is determined by employing one or more transducers to scan the profile of the cam and processing the signals so obtained. This way of operating may lead to inexact evaluations as it bases itself on not very sophisticated processings of data that result from measurement of a profile to be checked and therefore are presumably affected by error.
Thus the aforementioned methods and devices do not guarantee a precise measurement of a cam profile, and do not enable a rapid and strictly accurate inspection of an entire camshaft.
Another known method for checking the profile of a cam of a camshaft is described and illustrated in English patent application GB-A-2148007. While the shaft is axially rotated, signals relating to radial dimensions of the cam and signals relating to the angular position of the cam are sent in a continuous way to a processor unit that matches the values of one and the other type and compares these values with known nominal values of the cam profile. Before the actual measurement cycle starts, an initial angular reference value is determined, by inspecting the profile of the cam to be checked and processing the collected data. Even though this method involves relatively simple mechanical and control means for its embodiment, it requires the storing and the processing of an enormous amount of data. Furthermore, only one cam at a time can be measured and the changeover from one cam to another requires the repeating of operations like the determining of the initial angular reference point of the new profile. This initial reference may even turn out to be inexact as it greatly depends on possible errors in the actual dimensions of the cam.